Photovoltage Detector

ABSTRACT

A method and system for detecting and monitoring a temporal and spatial distribution of a light beam are provided. A semiconductor substrate ( 120 ) having a given doping concentration range is partially exposed to an incident laser beam ( 150 ). Each part of the semiconductor structure ( 120 ) which is exposed to the laser beam is provided with an electrical contact ( 145 ), which outputs a voltage which is directly related to the optical power or energy incident on the exposed area. The thermo-voltage is produced by the laser induced thermal gradients. The sensitivity and inter-pixel cross-talk is determined by pixel pitch, doping concentration and window opening ( 110 ). Depending of the design, each pixel might be sensitive to the temporal variation of the laser beam.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to methods and systems for measuringand/or monitoring the temporal and spatial profile of high power laserbeams in high optical power applications.

BACKGROUND OF THE INVENTION

Conceiving detectors and/or cameras, which can monitor the spatial andtemporal distribution of powerful CO₂ laser beams, is an important issuefor industrial CO₂ laser applications. Beam monitoring is needed forpulsed laser applications, i.e. with a pulse width ranging frompicoseconds to milliseconds, as well as for continuous wave (CW)applications. Beam diagnostics prevent laser based industrial processesof going unexpectedly down for some while. In general the laser beamquality deteriorates in time due to laser-induced damage of opticalcomponents in the setup or due to thermally induced misalignments in theset-up.

Two fundamental laser beam detection mechanisms can be distinguished:Energy detectors which respond to temperature changes generated byincident ration, e.g. incident IR radiation, through changes in materialproperties, and photon detectors which generate free electrical carriersthrough the interaction of photons and bound electrons. Energy detectorsare low cost and typically used in single detector applications.However, the simplicity of fabricating large 2D focal plane arrays insemiconductors has lead to the use of photon detectors in almost alladvanced IR detection systems. Examples of photon detectors are thequantum-well and quantum-dot infrared detectors, photoconductive andphotovoltaic detectors. Energy detectors contain two elements, anabsorber and a thermal transducer. Examples are pyroelectric detectors,ferro-electric detectors and thermistors, such as e.g. bolometers.Pyroelectric and ferroelectric detectors comprise a polarized materialwhich when subjected to changes in temperature changes its polarization.In thermistors the resistance of the elements varies with temperature.

In the continuous wave (CW) regime, problems arise already with existingcommercial detectors, such as photo-electromagnetic (PEM), pyroelectricdetectors (PE), for beam powers larger than about 30 W. The sensitivityof existing CW detectors (PE, PEM), . . . can be quite high at lowoptical powers, i.e. several thousands of V/W, but at higher opticalinputs two phenomena yield a strong degradation of the detectors output.At first there is tremendous non-linearity in the electrical output whenthe optical input increases, the sensitivity can degrade with severalorders of magnitude when the input changes with some orders ofmagnitude, hence these detectors have a very limited dynamic range.Secondly, beyond some optical power level thermal damage occurs due totheir limited thermal evacuation and due to the very local absorptionprocess. In pulsed applications the optical power density can be veryhigh and commercial detectors completely fail to detect these opticalpower levels. At high optical power levels, i.e. larger than 1 MWcm⁻²,there is no viable available detector/camera for pulsed applications asthe existing commercial devices loose their sensitivity at high opticalpower levels, i.e. their sensitivity drops by several orders ofmagnitude.

In general it can be said that most of the aforementioned detectors areused to collect the energy emitted by all kind of heated objects. Thedetected energy is translated into imagery showing the energy differencebetween objects, thus allowing an otherwise obscured scene to bevisualised. Typical applications of these cameras are for thermalimaging, night vision camera, automotive, fire fighting, electronicsthermal inspection, and industrial process monitoring. Such detectorstypically cannot be used for higher power applications. Consequently,there is a need for the development of novel detector types for themarket of laser beam profiling, withstanding much higher power levels.Several systems, used for positioning or measuring power output oflasers are already known.

U.S. Pat. No. 3,624,542 describes a method for checking the luminouspower output of a laser comprising a beam splitter. A portion of thelaser beam is directed towards a temperature sensitive device. Thedevice is made of a heat-receiving cone which collects a fraction of thebeam and directs it to a thermocouple setup. A variation in the laseroutput leads to a variation in electrical output, which allowscontinuously monitoring and providing feedback for the output of alaser. The document nevertheless only describes a method for controllingthe power output of a light beam, it does not allow to control a lightbeam profile as it does not provide spatial information. No extensiontowards array configuration is possible.

Patent application US 2001/0042831 A1 describes a photon detectorwhereby thin films are deposited on a substrate and whereby theabsorption, detection and removal of the generated photon heat isperformed in thin films. Nevertheless for real high power systems, thepossibility that thermal and mechanical damage occurs is significantlylarge. In a similar way, patent application US 2003/0164450 A1 describesa method and system for detecting thermal radiation, whereby thin filmdetector elements are used. Furthermore, US 2003/0164450 A1 describesthe use of focussing elements which also is not advantageous if highpower laser systems are used.

U.S. Pat. No. 4,243,888 describes a system for laser beam alignmenthaving a detector disc which is disposed in the laser beam path. Theincident laser beam creates thermoelectric effects in the detector whichyields voltage signals induced by pulsed heat diffusion. The voltagesignals are collected in a short period and integrated to determine theposition of the beam and to correct this position if necessary. U.S.Pat. No. 4,243,888 does not provide a device or method for studying theprofile of the laser beam light.

The problems with the prior art methods and systems is that they do notallow detection in real high power laser systems as they incorporatethin film detectors or detection systems positioned directly in thelaser beam path. Furthermore, most of the systems have the disadvantagethat they only allow to monitor the output power of the laser or providea means for aligning the laser beam.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method and systemfor monitoring/recording the optical power or energy of laser beams withits temporal evolution and monitoring/recording the spatial profiles ofthe laser beam with their temporal evolution.

The above objective is accomplished by a method and device according tothe present invention.

The present invention relates to a detector for characterising a highpower light beam, said detector comprising at least one pixel, whereineach of said at least one pixel comprises

-   -   a doped semiconductor substrate adapted to substantially absorb        an incident light beam, being at least a fraction of said high        power light beam, over an absorption distance L, said        semiconductor substrate having thermoelectric properties, said        doped semiconductor substrate being doped over at least part of        the thickness of the semiconductor substrate,    -   means for partly covering the doped semiconductor substrate such        that a window opening is provided at a first side of the doped        semiconductor substrate for receipt of said incident light beam        in said doped semiconductor substrate    -   means for measuring an electrical signal induced by a        thermoelectric effect in said doped semiconductor substrate        comprising a first electrode at said first side of said doped        semiconductor substrate and a reference electrode, said        reference electrode positioned outside an absorption volume        determined by said window opening and said absorption distance        L.        The doped semiconductor may be doped over the full thickness of        the semiconductor substrate. An absorption distance L may be the        minimum of the absorption length L_(abs) and the thickness of        the absorbing part of the substrate. The absorption principle        used in the detector may be a free carrier absorption process.

The detector may comprise a means for reducing cross-talk. The means forreducing the cross-talk may be a means for reducing the cross-talkbetween neighbouring pixels of said plurality of pixels adapted forreducing the cross-talk at least below a 5% acceptance level. The meansfor reducing the cross-talk may be a means for reducing the cross-talkbetween any pixel electrode of said plurality of pixels and thereference electrode, adapted for reducing the cross-talk at least belowa 5% acceptance level.

The means for reducing the cross-talk comprises a cooling channelbetween at least two pixels. The cooling channel has a depth which is atleast 15%, preferably at least 25%, more preferably at least 50% of thepenetration depth L_(abs).

The means for reducing cross-talk may be positioned outside the planedetermined by said semiconductor substrate.

The means for reducing cross-talk may comprise a heat sink provided inthermal contact with said semiconductor substrate. Furthermore areflecting plane may be provided adjacent the heat sink.

The means for reducing cross-talk may comprise a reflector materialpositioned around the window opening.

The means for partly covering the doped semiconductor substrate and saidabsorption distance L may be adjusted so that$\frac{P_{pixel} \cdot L}{S_{pixel}}$is in the range 0.1 to 100 wherein P_(pixel) is the perimeter of thepixel window and S_(pixel) is the surface area of the pixel window.

Characterising a high power light beam may be detecting any of a spatialand/or temporal intensity profile, a spatial and/or temporal energyprofile, a spatial and/or temporal energy density profile or a spatialand/or temporal power profile of the beam.

Said first electrode may define at least the perimeter of the windowopening. Said first electrode may comprise at least one elongateelectrode extending over the window opening. Said first electrode may beseparated at least partly from said doped semiconductor substrate bymeans of an insulating layer. Said first electrode furthermore maycomprise a conductor line and a bonding pad, deposited on top of saidinsulating layer.

The adjustment of the absorption distance L may be performed byadjusting the doping level of said doped semiconductor substrate.

In a detector comprising a plurality of pixels, each pixel having apixel window with an average pixel window width w, the pixels beingseparated by at least an interpixel pitch P, said interpixel pitch P maybe between 1 and 10 times the average pixel window width w, preferablybetween 1.5 and 8 times the average pixel window width w, morepreferably between 1.5 and 5 times the average pixel window width w,even more preferably between 1.5 and 4 times the average pixel windowwidth.

Operation of said detector may be selected between an intensitymeasuring mode, for measuring the intensity of a light beam, an energymeasuring mode, for measuring the energy of a light beam and a powermeasuring mode, for measuring the power of a light beam by adjusting theabsorption distance L and the window opening used.

At each first electrode a switch and a storage means may be provided,for temporary storing the pixel information.

The detector furthermore may comprise a read-out electronic circuitryadjusted to sample at regular moments the time evolution of theelectrical detector outputs and convert the sampled analogue voltagesinto digital signals. Said read-out electronic circuitry may be adjustedfor detecting the maxima in time of the detector outputs and convertthese sampled analogue voltages into digital signals.

Said second electrode may be positioned at a second side of thecompletely doped semiconductor substrate, the first and second sidebeing opposite with respect to each other.

The invention relates to a detector for characterising a high powerlight beam, said detector comprising at least one pixel, wherein each ofsaid at least one pixel comprising a doped semiconductor substrateadapted to substantially absorb an incident light beam, being at least afraction of said high power light beam, over an absorption distance L,said semiconductor substrate having thermoelectric properties means formeasuring an electrical signal induced by a thermoelectric effect insaid doped semiconductor substrate, said means partly covering the dopedsemiconductor substrate such that a window opening is provided at afirst side of said doped semiconductor substrate for receipt of saidincident light beam in said doped semiconductor substrate and said meanscomprising a first electrode at said first side of said dopedsemiconductor substrate and a reference electrode, positioned outside anabsorption volume determined by said window opening and said absorptiondistance L. The means for partly covering the doped semiconductorsubstrate and said absorption distance L may be adjusted so that$\frac{P_{pixel} \cdot L}{S_{pixel}}$is in the range 0.1 to 100 wherein P_(pixel) is the perimeter of thepixel window and S_(pixel) is the surface area of the pixel window. Theabsorption length L_(abs) for the light beam in said doped semiconductorsubstrate may furthermore be adjusted to keep the temperature of thesurface of said doped semiconductor substrate in said window openingbelow a maximum temperature, said absorption length L_(abs) beingbounded by${K \cdot \frac{t_{pulse} \cdot W_{0}}{C \cdot L_{abs}}} < T_{\max}$with K a proportional factor, t_(pulse) being the pulse width, W₀ beingthe incident laser intensity and C being the heat capacity of the dopedsemiconductor substrate. The absorption distance L for the light beam insaid doped semiconductor substrate may furthermore be adjusted to keepthe temperature of the surface of said doped semiconductor substrate insaid window opening below a maximum temperature, said absorption lengthL_(abs) being bounded byT _(surface,max) =W ₀min{L _(beam) ,L _(pix)}²/((L _(abs)+min{L _(beam),L _(pix)})χ)

with K a proportional factor, W₀ being the incident laser intensity andL_(beam) and L_(pix), are the laser beam width and the window width of apixel element. The proportional factor K lies in the range 0.1 to 10,preferably in the range 0.3 to 7, more preferably in the range 0.5 to 5,even more preferably in the range 0.5 to 3. Characterising said highpower light beam may be any of detecting a spatial intensity profile, aspatial energy profile, a spatial energy density profile or a spatialpower profile of the high power light beam. The first electrode may bedefining at least the perimeter of the window opening. The firstelectrode furthermore may comprise at least one elongate electrodeextending over the window opening. The second electrode may bepositioned at a second side of the doped semiconductor substrate, thefirst and second side being opposite with respect to each other. Thefirst electrode is separated at least partly from said dopedsemiconductor substrate by means of an insulating layer. The firstelectrode furthermore may comprise a conductor line and a bonding pad,deposited on top of said insulating layer. A reflective top layer may beprovided outside said window opening. The reflective top layer may beprovided on top of the first insulating layer and complementary isolatedfrom all the metal conductor lines. A second insulating layer incombination with a reflective top layer may be provided on top of thefirst insulating layer and all the metal conductor lines, except on thebonding pads and detector openings. A third set of electrical contactsmay be provided, covered by a reflective or absorptive layer, such thatlight is not incident in the direct neighbourhood of these contacts suchthat this set of contacts sense the optically induced temperaturedistribution of these parts of the substrate which are not directlyexposed to the incident laser beam. The adjustment of the absorptiondistance L may be performed by adjusting the doping level of said dopedsemiconductor substrate. The detector may comprise a plurality ofpixels, each pixel having a pixel window with an average pixel windowwidth w, the pixels being separated by at least an interpixel pitch P,wherein said interpixel pitch P may be between 1 and 10 times theaverage pixel window width w, preferably between 1.5 and 8 times theaverage pixel window width w, more preferably between 1.5 and 5 timesthe average pixel window width w, even more preferably between 1.5 and 4times the average pixel window width. The interpixel pitch P may besufficiently large to prevent significant cross-talk between thedifferent pixels. Between at least two of said plurality of pixels acooling channel may be provided. The cooling channel may have a depthwhich is at least 15%, preferably at least 25%, more preferably at least50% of the absorption distance L. The detector furthermore may comprisea means for reducing the cross-talk between neighbouring pixels of saidplurality of pixels adapted for reducing the cross-talk with a factor of5, preferably with a factor of 15, more preferably with a factor of 25.The amount of cross-talk thereby is expressed relative to the measuredsignal in a neighbouring pixel. The means for reducing the cross-talkmay be the cooling channel in between pixels, the reflective film on topof non-pixel areas and the means for covering the pixel area forreducing the fill factor and a heat sink at the bottom of the substrate.Operation of said detector may be selected between an intensitymeasuring mode, for measuring the intensity of a light beam, an energymeasuring mode, for measuring the energy of a light beam and a powermeasuring mode, for measuring the power of a light beam by adjusting theabsorption distance L and the window opening used. The plurality ofpixels may be arranged in an array. The plurality of pixels is arrangedin a m×n matrix, having m columns and n rows of pixels. At each firstelectrode a switch and a storage means may be provided, for temporarystoring the pixel information. The detector system may furthermorecomprise a read-out electronic circuitry adjusted to sample at regularmoments the time evolution of the electrical detector outputs andconvert the sampled analogue voltages into digital signals. The read-outelectronic circuitry may be adjusted for detecting the maxima in time ofthe detector outputs and convert these sampled analogue voltages intodigital signals. The light penetration depth for the optical spectrumunder consideration, dictated by the pulse width and the window opening,may be such that the maximum allowable lattice temperature inside thesubstrate at the maximum optical power density of the incident laserbeam is not reached. The electrical signal measured by the means formeasuring an electrical signal may be a voltage, generated between afirst contact of a first set of electrodes and a second contact of asecond set of electrodes and related to the optically inducedtemperature profile between the said contacts due to an opticalabsorption process. Thus, a doped semiconductor substrate withgeometrical constraints and having thermoelectric properties is used toconvert the beam parameters of high optical power laser beams intoelectrical signals.

The invention also relates to a system for monitoring the output of alight beam producing means comprising a light beam sampling means and adetector, whereby said light beam sampling means is adjusted to splitthe light beam in a first small fraction and a second large fraction,whereby said first small fraction of said light beam is directed towardsa detector as described above. The beam sampling means may be a mirror,said mirror being rotatably mounted as to split the light beam atregular periods.

The invention furthermore relates to a method for monitoring the profileof a light beam, the method comprising the steps of splitting said lightbeam into a first fraction for monitoring and a second fraction used foran application, directing said light beam to a detector, measuring withsaid detector at several points in the cross section of the fraction ofthe light beam, obtaining a plurality of electronic signals,representing the spatial intensity profile of said light beam. Measuringwith said detector at several points in the cross section of thefraction of the light beam may be performed by subsequently shifting atleast one single pixel detector as described above, to another point inthe cross-section of the fraction of the light beam and recording ameasurement. Measuring with said detector at several points in the crosssection of the fraction of the light beam may be performed by measuringsimultaneously with a multiple pixel detector as described above,several points in the cross-section of the light beam. For the lightbeam having a pulse width and the detector being able for measuring inintensity measuring mode, in energy measuring mode or in power measuringmode, the method may comprise setting a pulse width of said light beamfor measuring in intensity measuring mode, in energy measuring mode, inenergy density mode or in power measuring mode. In the intensitymeasuring mode, the output of the detection system may correspond withthe intensity of the light beam, in energy measuring mode, the output ofthe detection system may correspond with the energy of the light beam,in energy density measuring mode, the output of the detection system maycorrespond with the energy density of the light beam and in powermeasuring mode, the output of the detection system may correspond withthe power of the light beam.

The invention furthermore relates to a detector for characterising ahigh power light beam, said detector comprising at least one pixel, eachof said at least one pixel comprising a doped semiconductor substrateadjusted to substantially absorb an incident light beam, being at leasta fraction of said high power light beam, over an absorption distance L,said semiconductor substrate having thermoelectric properties and meansfor measuring an electrical signal induced by a thermoelectric effect insaid doped semiconductor substrate, said means furthermore partlycovering the doped semiconductor substrate such that a window opening isprovided for receipt of said incident light beam in said dopedsemiconductor substrate wherein the size of said window opening and saidpenetration depth L_(abs) are adjusted so that$\frac{P_{pixel} \cdot L}{S_{pixel}}$is in the range 0.1 to 100, wherein P_(pixel) is the perimeter of thepixel and S_(pixel) is the surface area of the pixel. The differentfeatures described in the dependent claims and in the detectorsdescribed above can, where appropriate, be applied to this detector.

The invention furthermore relates to a detector for characterising ahigh power light beam, said detector comprising at least one pixel,wherein each of said at least one pixel comprising a doped semiconductorsubstrate adapted to substantially absorb an incident light beam, beingat least a fraction of the high power light beam, over an absorptiondistance L, said semiconductor substrate having thermoelectricproperties and a means for measuring an electrical signal induced by athermoelectric effect in said doped semiconductor substrate comprising afirst electrode at a first side of said doped semiconductor substrateand a reference electrode at a second side of the doped semiconductorsubstrate, the first and second side being opposite with respect to eachother, said means furthermore partly covering the doped semiconductorsubstrate such that a window opening is provided at the first side ofsaid doped semiconductor substrate for receipt of said incident lightbeam in said doped semiconductor substrate.

The methods, detectors and detection systems as described above also maybe applied for characterising high power radiation beams and are notrestricted to high power light beams.

It is an advantage of the present invention that the detection systemand method can be used for a wide range of laser systems, reaching fromvery short pulsed laser systems with pulses in the range of nanoseconds,to continuous wave laser systems.

It is an advantage of specific embodiments of the present invention thatthe thermal cross talk between pixels can be limited to an acceptablelevel determined by the user.

It is furthermore an advantage of the present invention that selectioncan be made of the type of measuring mode of the detection system, byadjusting the pixel configuration and/or the properties of the lightbeam.

Although there has been constant improvement, change and evolution ofdevices and methods in this field, the present concepts are believed torepresent substantial new and novel improvements, including departuresfrom prior practices, resulting in the provision of more efficient,stable and reliable devices of this nature.

The teachings of the present invention permit the design of improvedmethods and apparatus for measuring the optical power or energy of highpower laser beams, with their temporal evolution and/or their spatialprofile.

These and other characteristics, features and advantages of the presentinvention will become apparent from the following detailed description,taken in conjunction with the accompanying drawings, which illustrate,by way of example, the principles of the invention. This description isgiven for the sake of example only, without limiting the scope of theinvention. The reference figures quoted below refer to the attacheddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic overview of a system using the detectionprinciple according to embodiments of the present invention.

FIG. 2 illustrates some free carrier absorption mechanisms in amulti-valley semiconductor, as used in embodiments of the presentinvention.

FIG. 3 illustrates the equivalent scheme of a semiconductor baseddetector, according to an embodiment of the present invention.

FIG. 4 a, FIG. 4 b, FIG. 4 c show a top view and a correspondingcross-sectional view for different lay-outs of the single pixeldetector, according to embodiments of the present invention.

FIG. 4 d shows a top view of another lay-out of a single pixel detector,according to embodiments of the present invention.

FIG. 5 a illustrates a partially absorbing pixel with a heat sinkconnected at the bottom of the substrate, according to embodiments ofthe present invention.

FIG. 5 b illustrates a graph of the maximum surface temperature changeas a function of the distance to the incident beam for detectors withand without heat sink, according to embodiments of the presentinvention.

FIG. 5 c illustrates a detector having a semiconductor substrateabsorbing in a zone with a thickness smaller than the substratethickness, according to embodiments of the present invention.

FIG. 5 d illustrates the temperature increase as a function of theabsorption length in partially absorbing substrates, according toembodiments of the present invention.

FIG. 6 a is a schematic overview of a one dimensional array of detectorpixels, each of them provided with a pad for external connection,according to an embodiment of the present invention.

FIG. 6 b shows a top view and a corresponding cross-sectional view fordifferent lay-outs of a single pixel detector with aprotective/reflecting top metal layer, according to an embodiment of thepresent invention.

FIG. 7 a shows a top view and corresponding cross-sectional views for adetector lay-out with a reflective/protective layer complementary to themetallization pads, according to an embodiment of the present invention.

FIG. 7 b shows a top view of an alternative design for a detectorlay-out with a reflective/protective layer complementary to themetallisation pads, according to an embodiment of the present invention.

FIG. 8 shows a top view and corresponding cross-sectional views for adetector lay-out with a reflective/protective layer complementary to themetallisation pads and isolated from each other, according to anembodiment of the present invention.

FIG. 9 shows a top view and a corresponding cross-sectional view for twopixels from a one dimensional line array for detection in the energysensing regime, according to an embodiment of the present invention.

FIG. 10 shows a top view and a corresponding cross-sectional view fortwo pixels from a one dimensional line array for detection in the powersensing regime, according to an embodiment of the present invention.

FIG. 11 a is an illustration of the time evolution of the temperature atvarious depths inside a substrate at the centre position of a detectorpixel according to an embodiment of the present invention.

FIG. 11 b is an illustration of the corresponding temperature changes atvarious depths inside the substrate, for a detector pixel according toan embodiment of the present invention.

FIG. 12 a, FIG. 12 b and FIG. 12 c show the time evolution of thetemperature at various distances from the centre of the pixel on the topside of the substrate for the energy sensing regime (FIG. 12 a), themixed energy-power regime (FIG. 12 b) and the power sensing regime (FIG.12 c) according to embodiments of the present invention.

FIG. 13 a and FIG. 13 b show an illustration of the cross-talk versusthe distance from the centre of the pixel for various dopingconcentrations with and without etched air gaps, for the energy sensingregime (FIG. 13 a) and the power sensing regime (FIG. 13 b) according toembodiments of the present invention.

FIG. 13 c shows an illustration of the cross-talk reduction versus thedepth of the etch air gap according to embodiments of the presentinvention.

FIG. 14 a and FIG. 14 b show a schematic illustration of a twodimensional array of detector pixels (FIG. 14 a) and an equivalentscheme of the pixel electronics (FIG. 14 b) for a detector systemaccording to an embodiment of the present invention.

FIG. 15 shows a block diagram of a read-out scheme for a detector systemaccording to an embodiment of the present invention.

FIG. 16 shows a block diagram of an alternative read-out scheme for adetector system according to an embodiment of the present invention.

FIG. 17 shows a possible setup of a beam sampling system according to anembodiment of the present invention.

FIG. 18 shows an alternative setup of a beam sampling system accordingto an embodiment of the present invention.

In the different figures, the same reference signs refer to the same oranalogous elements.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The present invention will be described with respect to particularembodiments and with reference to certain drawings but the invention isnot limited thereto but only by the claims. The drawings described areonly schematic and are non-limiting. In the drawings, the size of someof the elements may be exaggerated and not drawn on scale forillustrative purposes.

Furthermore, the terms first, second, third and the like in thedescription and in the claims, are used for distinguishing betweensimilar elements and not necessarily for describing a sequential orchronological order. It is to be understood that the terms so used areinterchangeable under appropriate circumstances and that the embodimentsof the invention described herein are capable of operation in othersequences than described or illustrated herein.

Moreover, the terms top, bottom, over, under and the like in thedescription and the claims are used for descriptive purposes and notnecessarily for describing relative positions. It is to be understoodthat the terms so used are interchangeable under appropriatecircumstances and that the embodiments of the invention described hereinare capable of operation in other orientations than described orillustrated herein.

It is to be noticed that the term “comprising”, used in the claims,should not be interpreted as being restricted to the means listedthereafter; it does not exclude other elements or steps. Thus, the scopeof the expression “a device comprising means A and B” should not belimited to devices consisting only of components A and B. It means thatwith respect to the present invention, the only relevant components ofthe device are A and B.

Similarly, it is to be noticed that the term “coupled”, also used in theclaims, should not be interpreted as being restricted to directconnections only. Thus, the scope of the expression “a device A coupledto a device B” should not be limited to devices or systems wherein anoutput of device A is directly connected to an input of device B. Itmeans that there exists a path between an output of A and an input of Bwhich may be a path including other devices or means.

Although the invention has been described for characterisation of highpower light beams, the methods and systems also can be applied forcharacterisation of a large number of other types of radiation.

In a first embodiment, the invention relates to a method of detectingand a detection system for measuring the optical power of a high powerincident radiation beam. FIG. 1 illustrates a cross-sectional view of aportion of an opto-electronic device 100 for a first embodimentconstructed according to the principles of the present invention. Thedevice 100 comprises a window opening 110 on a doped semiconductorsubstrate 120 exposed to an incident radiation beam 150 such as a lightbeam, especially an incident bundled light beam or incident laser beam.The radiation beam may be a pulsed or continuous radiation bean. Theincident light may be at any wavelength, e.g. optical, far or near IR orUV wavelengths. Normally, incident light 150 enters the device 100 fromair and is exponentially absorbed in the substrate 120. This opticalabsorption generates heat or a temperature difference in the substrate120. As the optical beam propagates through the sample, the opticalpower deeper inside the substrate exponentially decreases due to itslocal absorption, which leads to decreased local temperature rise deeperinside the substrate 120. The induced temperature gradient in theelectron/hole and/or phonon temperature distribution between a pointclose to the incident illuminated area and a point sufficiently remotefrom the incident illuminated area produces a voltage output, i.e. avoltage step, due to the Seebeck effect.

Voltages generated in the device can be measured using output electrode140, also referred to as reference electrode, which can be e.g. atground potential, and output electrode 145. To prevent short circuitingwith the doped semiconductor substrate 120, an insulating layer 130 isprovided.

The doped semiconductor substrate 120 may be any semiconductorsubstrate. Single crystal semiconductor substrates typically have athermo electromotive force coefficient, also called Seebeck coefficient,in the order of 1 mV/K. The semiconductor substrate 120 in the presentinvention may be, but is not limited to, e.g. GaAs, InAs, InP, InSb,GaSb. A typical thickness of such a semiconductor substrate 120 may bebetween 100 and 500 μm, although not being limited thereto.

The conductive electrodes 140 and 145 can be any electrode, such as ametal electrode. Preferably, the electrode is made of a relatively heatresistant material, e.g. the melting point of the electrode materialpreferably is high. The metal electrodes typically have a thickness of0.1 micron to a few microns.

The incident light 150 can be absorbed in the semiconductor substrate120 in several ways, i.e. through vibrational excitation or electronicexcitation, e.g. free carrier absorption, via chromophores or defects,by band-to-band transitions. In most cases, the most important lightabsorption mechanism will be based on free carrier absorption. Lightabsorption by free carrier absorption can take place only by interactingwith a third “particle”, such as phonons, impurities and otherimperfections. Among the various free carrier absorption process one candistinguish between the following main mechanisms: intravalley andequivalent and non-equivalent intervalley absorption mechanisms. Themost important intervalley mechanisms at room temperature are (hot)polar optical phonon and impurity assisted absorption. Minorcontributions are come from acoustic and non-polar assisted absorption.Intervalley absorption mechanisms are assisted by intervalley phonons.For a multi-valley semiconductor such as e.g. GaAs, one needs to takeinto account the following intervalley transfers: the non-equivalenttransfers such as Γ-L,

-Γ; Γ-X,

-Γ; L-X, X-L and the equivalent transfers: L_(i)-L_(j), X_(r)-X_(s).Some of the intravalley transitions 152, 154 and some of the intervalleytransitions 156 are illustrated in FIG. 2. It will be obvious for aperson skilled in the art that the detector principle used and thus thedetector system of the present invention is not limited by the type oflight absorption processes that occurs. The doping level of thesemiconductor substrate 120 is preferably adjusted such that theabsorption coefficient is optimized for relevant application of thepresent invention. The selection of the doping level, in accordance withthe absorption coefficient to be obtained can be easily performed by aperson skilled in the art. Depending of the optical power level andpulse width, a typical range of the absorption length is between 20 and1000 μm. The smallest absorption lengths are chosen for applicationswith the lowest opto-thermal impact and vice versa. For e.g. n-type GaAsand an infrared wavelength of 10.6 μm the corresponding dopingconcentration is in the range 1×10¹⁷ and 5×10¹⁸ cm⁻³. The largest dopingconcentration corresponds to the smallest absorption length.

The exponential decay of the optical power along the propagationdirection normal to the substrate-air interface is schematicallyillustrated in FIG. 1 by curve 160. At each co-ordinate z along thepropagation direction Z a local electron temperature in case of freeelectron absorption and phonon temperature is established due to theabsorption process. It is strongly recommended that only one type ofcarrier is present, either only electrons or only holes, as the presenceof both type of carriers leads to a drastically decrease of thesensitivity of the detector. In the following there is focussed onn-type semiconductors having free electrons. Nevertheless, the sameholds for p-type semiconductors having free holes, the invention thusnot being limited to n-type semiconductors. Initially only the electrontemperature rises. After a short time when there is exchange between theelectrons and phonons, also the phonon temperature rises. Theestablished 3D temperature profiles inside the substrate are aconsequence of drift and diffusion processes of the particles and of theexistence of thermal gradients. The free electron temperature and phonontemperature distribution are graphically illustrated by curves 170 and180, respectively. For times smaller than the phonon temperature risetime, the free electron and phonon temperature differ from each other.After equilibrium has been reached both electron temperature and phonontemperature are equal. For each of the particles, i.e. for the electronsand the phonons, a temperature gradient is induced by the localabsorption process. A differential thermo-electromagnetic force voltageis generated due to the gradients in the electron temperature profile aphenomenon called the Seebeck effect. When no electrical current canflow inside the substrate a thermo-voltage is induced by the freeelectron heating. In the simplest case this can be expressed as follows:$\begin{matrix}{{{\sigma\left( T_{e} \right)}{\nabla\left( {\varphi - \frac{\varsigma}{q}} \right)}} = {{\eta\left( T_{e} \right)}{\nabla T_{e}}}} & \lbrack 1\rbrack\end{matrix}$

where σ(T_(e)) is the electrical conductivity at temperature T_(e),φ−ζ/q is called the electrochemical potential, having a pure electricalpart φ and a pure chemical part ζ/q with q the electron charge, η(T_(e))is the thermoelectric coefficient at temperature T_(e). ∇T_(e) is theelectron temperature gradient induced by the local absorption process.

The voltage across the sample can then be expressed in the followingway: $\begin{matrix}{V = {\int_{T_{e,1}}^{T_{e,2}}{{\alpha\left( T_{e} \right)}\quad{\mathbb{d}T_{e}}}}} & \lbrack 2\rbrack\end{matrix}$

where α(T_(e))=η(T_(e))/σ(T_(e)). represents the differentialthermo-electromotive force (e.m.f.) coefficient or Seebeck coefficient.It is important to notice that this Seebeck coefficient needs to becalculated on the basis of all the conduction band valleys contributingto the heating process. In the case of a p-type semiconductor all thevalence band valleys (heavy-hole, light-hole and spin-split off band)need to be taken into account.

As an example, the total value of the coefficient of differentialthermo-electromotive force α in the case of multi-valley free electronabsorption is given by the following expression:α=(α_(Γ)σ_(Γ)+α_(L)σ_(L)+α_(X)σ_(X))/(σ_(Γ)+σ_(L)+σ_(X))  [3]where σ_(Γ,L,X) is the electron conductivity andα_(Γ,L,X is the coefficient of differential thermo electromotive force in Γ, L and X valley, respectively.)

In order to solve equation [2], it is necessary to know theelectron/hole and phonon temperature distribution. To find thedistribution of the phonon temperature T_(L) in the substrate, theequation of the lattice thermoconductivity is solved $\begin{matrix}{{C\frac{\partial T_{L}}{\partial t}} = {{\chi\left( {\frac{\partial^{2}T_{L}}{\partial x^{2}} + \frac{\partial^{2}T_{L}}{\partial y^{2}} + \frac{\partial^{2}T_{L}}{\partial z^{2}}} \right)} + {P\left( {T_{e},T_{L}} \right)}}} & \lbrack 4\rbrack\end{matrix}$where t is time, x, y, z are the space coordinates, χ is thethermoconductivity coefficient, C is the heat capacity coefficient andP(T_(e), T_(L)) is the power transferred to the lattice by the heatedelectron gas for electron temperature T_(e).

Neglecting the effect of the electron/hole thermoconductivity we canwrite down the following relation:P(T _(e) ,T _(L))=α_(e) W  [5]

where α_(e) is the electromagnetic wave absorption coefficient forelectrons, and W is the electromagnetic wave intensity inside thesubstrate. The thermo-voltage generated by the incident laser beam canbe sensed between output 140, which is at ground potential, and output145 respectively, as indicated in FIG. 3. It is to be noted that theoutput electrode 140, also referred to as reference electrode may bepositioned outside an absorption volume of said detector. As thesubstrate 120 is doped and thus may have conducting properties, aninsulating layer 130 is provided to prevent a short-circuit between theoutput connector conductive layer 145, e.g. a metal layer 145 and thesubstrate 120. The material used for the insulating layer 130 typicallymay be an undoped semiconductor layer quasi lattice-matched to thesubstrate (e.g. an AlGaAs layer on a GaAs substrate or an InGaAs layeron an InP substrate) or an Al₂O₃ layer, a Si₃N₄ layer, . . . .

By calculating the output voltage V versus the light intensity W insidethe substrate, one can derive the sensitivity S_(heat) expressed inmV.cm²/kW of this detector. A more detailed deduction of the aboveequations and a more thorough description of the coefficients in theseequations can be found in the description of the correspondingquantum-mechanical model for free electron absorption as described by G.Shkerdin, J. Stiens and R. Vounckx e.g. in European Physics Journal,Applied Physics, vol. 19(1), pages 29-38 (2002), in European PhysicsJournal, Applied Physics, vol. 12(3), pages 169-180 (200), in Journal ofApplied Physics, vol. 85(7), pages 3807-3818 (1999), in Journal ofApplied Physics, vol. 85(7), pp. 3792-3806 (1999), and as described byE. Driessens, J. Stiens, G. Shkerdin, V. Kotov and R. Vounckx, inproceedings of THERMICS, 9^(th) Workshop on Thermal Investigations ofICs and Systems, pp. 145-148 (Aix-en-Provence, France 24-26 Sep. 2003).

A typical sensitivity obtained with the fast electron temperaturegradients is about 3.0 mV.cm²/MW, for electron temperature gradients inequilibrium with the phonon temperature gradient this sensitivity isabout 1000 times larger, but the detector response time is much slower.The equivalent circuit of the proposed detector is illustrated in FIG. 3by the open-circuit voltage V_(t,OC), directly connected with theoptical power or energy of the incident laser beam 150 depending on thepulse characteristics and the detector geometry. The internal resistanceof the circuit is indicated by resistance 195.

The voltage output from any contact reflects the optically inducedsurface temperature nearby said contact. The temporal evolution of thistemperature or corresponding voltage is determined by the substrate 120and the laser pulse characteristics.

The type of detector output, and consequently the information that canbe obtained, is determined by the ratio between the laser pulse durationand various time constants of the detector. The following time constantsof the detector of this invention are important: the electron heatingtime, the electron energy relaxation time τ_(e) and the lattice thermalresponse time τ_(sub).

The heating of the electrons is almost instantaneous with respect toindustrial applications. For example in the case of n-type GaAs, thisheating time constant is of the order of 100 fs. Hence the electrontemperature immediately follows the incoming light intensity. This timeconstant will not be further considered in this invention.

The electron energy relaxation time τ_(e) is not a constant value butdepends on material parameters such as the doping concentration andincident light intensity. For e.g. n-type GaAs, this time constant istypically of the order of a few ps. For incident laser pulses which arelonger than a few electron energy relaxation time constants, thedetector's voltage output will be directly related to the temporalevolution of the laser intensity. In this case the lattice temperatureis almost unaffected. When the laser pulse gradually gets longer theenergy of the electron gas is gradually transferred to the lattice ofthe doped semiconductor, such as e.g. the n-type GaAs lattice, whichresults in a gradually increasing lattice temperature. For a given laserpulse length both electron temperature and lattice temperature will havethe same order of magnitude. This is the regime of the mixedelectron-lattice effects. The pulse width for which both temperaturesbecome equal is called the τ_(,mix) and is in a first orderapproximation defined by equation [6]: $\begin{matrix}{\tau_{mix} = \frac{\tau_{e}C}{N \cdot k_{B}}} & \lbrack 6\rbrack\end{matrix}$This expression shows that this time constant τ_(,mix) is dependent onthe doping concentration N but not dependent on the light intensity.E.g. for doping concentrations in the range of 10¹⁸ cm⁻³, the pulsewidth for which both temperatures get equal, i.e. τ_(mix), is about 100ns. However, for high optical intensities the expression (6) for τ_(mix)should be corrected for nonlinear effects and reads then as shown byequation [7] $\begin{matrix}{\tau_{mix} = \frac{\delta\quad t_{e}C}{\alpha_{e}W}} & \lbrack 7\rbrack\end{matrix}$whereby δt_(e) is the increase of electron temperature due to the lightabsorption process. Numerical simulations show that within the dopingconcentration interval 0.7×10¹⁸-4×10¹⁸ cm⁻³, the value τ_(mix) can dropfrom the range 325-40 ns down to 100-10 ns when the intensity variesfrom a low level to about 100 MWcm⁻².

For pulses longer than τ_(mix) the detector behaves as an energy meter,i.e. the detector's voltage output is the integration of the incidentlaser intensity. This behaviour will continue as long as the pulse timeis smaller than the time constant related to the thermal response timeof the illuminated area. Pulse lengths approaching the thermal responsetime of the illuminated area will be gradually leading to slowing downof the lattice temperature increase due to the thermo-conductivity.

The thermal response time of the illuminated substrate area 110,annotated with τ_(sub) is described in the following expression [8]$\begin{matrix}{\tau_{sub} = \frac{\beta \cdot C}{\chi\left( {L_{abs}^{- 2} + {\min\left( {\frac{L_{pix}}{2},\frac{L_{beam}}{2}} \right)^{- 2}}} \right)}} & \lbrack 8\rbrack\end{matrix}$

wherein C is the heat capacity and χ is the heat conductivity of thesubstrate material. L_(abs), L_(beam) and L_(pix), are the absorptionlength inside the substrate, the laser beam width and the window widthof a pixel element, respectively. β is a geometrical numerical factor ofthe order of one and depends on the details of the pulse characteristicsand pixel geometry. In contrast to the first two time constants whichare only material dependent, the τ_(sub) is also dependent on thegeometry. It is to be noted that in order to increase the dynamic rangeof the detector, partially absorbing semiconductor substrates may beused. The dynamic range then is exponentially dependent on the ratiothickness of the absorbing zone/absorption length L_(abs). For pulsesshorter than the thermal response time of the illuminated substrate area110 but longer than τ_(mix), the maximum voltage output, and thus themaximum surface temperature T_(surface,max) is proportional to the pulsewidth t_(pulse) of the laser as given in expression [9] and isindependent on the pixel area.T _(surface,max) =K.t _(pulse) W _(o)/(CL _(abs))  [9]

The proportionality factor K thereby lies within the range 0.1 to 10,preferably within the range 0.3 to 7, more preferably between 0.5 and 5,even more preferably between 0.5 and 3. Here W_(o) is the intensity ofthe laser beam. One can show that the upper limit for this energyoperation regime is approximately given by the time constantτ_(erg)=τ_(sub)/γ. The factor γ is determined by numerical calculationsand varies roughly between 10 and 100. In order to extend the regime ofthe energy measurements, i.e. to make τ_(erg) as large as possible, oneneeds small τ_(sub) values in combination with small γ values which arereached for large absorption lengths and large pixel sizes.

The temperature can either be sensed in the centre of the pixel orfurther away from the centre of the pixel at a distance R. Temperaturessensed further away at least at a distance equal to the double of thepixel or beam radius away from the centre of the pixel area, e.g. with aperimeter electrode, are directly related to the total energy incidentinside the pixel area, hence these temperatures are proportional topulse width and laser beam area. The factor F(R) is a factor forexpressing the dependence on the radius of the perimeter electrode.T _(peri,R) ˜F(R).t _(pulse) P(CL _(abs))  [10]

Table 1 contains the relative temperature increase for both a centre anda perimeter electrode for laser beams which differ only in size, i.e.have a different radius. The pulses under consideration are 300 ns longincident on a sensor with diameter equal to 400 μm. The relativetemperature increase is expressed with respect to the smallest laserbeam. This table shows that the relative temperature increase in thecentre of the pixel is independent on the beam size whereas the relativetemperature increase at the perimeter is almost perfectly related to theratio of the area. TABLE 1 Radius (μm) Tc, rel Tp, rel Area Ratio 10 1 11 25 1.03 6.258 6.25 50 1.031 25.08 25 75 1.031 56.5 56.25 100 1.031105.2 100

In other words, by properly designing the top electrode used, differenttypes of information can be obtained. For pulse lengths between thetiming parameters τ_(mix) and τ_(erg), the obtained information for acentre contact of the pixels is the energy incident in the centre of thepixel whereas a perimeter contact, which is far away from the centre ofthe pixel allows to measure the total energy incident on the pixel. Theenergy measurement in the centre of the pixel allows, afterdifferentiating of the signal, to obtain information about the timeevolution of the intensity in the centre of the pixel. With this type ofcontact, the output for beams having the same maximum intensity butdifferent beam diameters will always be the same, while the measurementallows distinguishing beams having the same total energy but differentmaximum intensities. The total energy measurement obtained by use of theperimeter electrode allows obtaining, after differentiation of thesignal, information about the time evolution of the average intensity inthe pixel. The use of a perimeter electrode allows distinguishingbetween beams having the same maximum intensity but different totalenergy. For pulses much longer than the thermal response time τ_(sub),the maximum output is independent of the pulse width and can beestimated by the following expressionT _(surface,max) ˜W ₀min{L _(beam) ,L _(pix)}²/((L _(abs)+min{L _(beam),L _(pix)})χ)  [11]This regime, called the regime of power measurements is approximatelyvalid for pulses longer than τ_(pow)=γτ_(sub) sub where γ is the samefactor as for the energy measurements. In that sense γ is defined asfollows. $\begin{matrix}{\gamma = \sqrt{\frac{\tau_{pow}}{\tau_{erg}}}} & \lbrack 12\rbrack\end{matrix}$In order to extend the regime of the power measurements as much aspossible, i.e. to make τ_(pow) as small as possible, one needs thesmallest τ_(sub) values in combination with small γ values. The smallestvalues of τ_(pow) are reached for small absorption lengths incombination with small pixels. In the power-sensing regime the maximumtemperature is increasing with increasing pixel sizes. Hence for highoptical power measurements it is needed to limit the pixel size which isin favour for a maximum extension of the power regime towards smallpulses.

Similar as for beams having a pulse length between τ_(mix) and τ_(erg),incident beams having a pulse length larger than τ_(pow) can becharacterised in a different way, depending on the specific topelectrode design. Using a centre contact allows obtaining informationabout the local power incident on the centre of the pixel, wherebydifferentiating the signal further allows obtaining information aboutthe time evolution of the power in the centre of the pixel. For aperimeter contact, the total power of the incident beam is measured anddifferentiating allows to obtain information about the time evolution ofthe total power in the pixel. As a numerical example, a n-GaAs substratewith a doping concentration N=7×10¹⁷ cm⁻³ and a corresponding absorptionlength L_(abs)=294 μm and with a circular pixel with a radius of 25 μmand a reflective Au layer on top of a secondary insulator layer. One canderive that β=1.9 and γ=47. For all laser beams which are substantiallylarger than the pixel size, formula [8] can be applied to determine thetime response of the pixel, i.e. τ_(sub)=95 μs. For pulsed laser beamswhich are shorter than τ_(erg)=2 μs the detector pixel behaves like anenergy meter, when the laser beam is longer than 4 ms the voltage outputof the pixel is related to the temporal evolution of the optical powerof the laser beam. In the latter case, an array of pixels willeventually produce the spatio-temporal evolution of the optical power ofthe laser beam. In other words, the detection system and methodaccording to the present invention can be operated in 5 differentregimes, i.e. three individual regimes and two mixed regimes. The sensorelements can measure in a regime of temporal intensity profiling, aregime of temporal energy measurements, which leads afterdifferentiating to time to temporal intensity profiling, a mixed regimewherein the combination of intensity and energy measurements is made andwhereby detailed profiles only can be obtained with numerical models, aregime wherein temporal power measurements can be made and a mixedregime wherein energy and power measurements are combined, wherebydetailed profiles can only be retrieved with numerical models. Thespecific modes like energy measuring mode, power measuring mode andintensity measuring mode can be used either to directly obtain theproper quantity of the light beam that is to be studied, or acombination of these modes can be used to obtain a broad measurementrange, i.e. to obtain a system that can provide information about lightbeams with a very short pulse, i.e. in the range of picoseconds, tocontinuous wave lighting systems. It thereby is a specific advantage ofthe present invention that these measurements can be done for opticalhigh power systems, from about 1 Wcm⁻² up to a few GWcm⁻². Based on theabove equations, a detector system can either be adjusted, i.e. byadjusting for example the pixel configuration, such that a specificmeasuring mode can be used for a specific type of light beam, or thepulse width of the light beam can be adjusted to be able to measure itin a specific mode for a given detector system.

Persons skilled in the art will understand how to interpret detector andpixel outputs and their temporal behaviour based on the given detectordimensions (substrate material, window opening, absorption length) and afirst estimate of the beam characteristics.

In a second embodiment, several preferred configurations for the sensorelements as described in the first embodiment are described. Thesepreferred configuration are illustrated in FIG. 4 a, FIG. 4 b, FIG. 4 cand FIG. 4 d. Measurements typically are made between electricalcontacts 145 and a reference electrode 140, positioned outside anabsorption volume of the detector, e.g. but not limited to an electrodeat the bottom side of the detector. Ideally the metal contact 145 to thepixel should be minimal in size in order to have minimum disturbance onthe optical laser input. However, small contacts 145 can introduce largeserial resistances 195. The maximum resistance should not be bigger thanthe input resistance of the read-out electronics. In case bothresistances are equal the sensitivity of the detector is halved. In FIG.4 a the electrical contact 145 to the pixel is provided along theperimeter of the pixel. An insulating layer 130 is partially providedbetween the metal layer 145 and the doped semiconductor substrate 120.It will be obvious for a person skilled in the art that the shape of thepixel does not need to be a square. It can also be a circular,rectangular or even arbitrarily shaped pixel window. When the laser beamand the pixel are optimally aligned, one can expect that the highesttemperature gradient and hence voltage output is established in thecentre of the pixel. Hence in a preferred configuration as illustratedin FIG. 4 b the metal contacts are in the form of one or more fingers146 a, 146 b, i.e. an elongated layer having a large length/width ratio,pointing to the centre of the pixel window. The more fingers 146 a, 146b one provides during manufacturing of the detecting device, the smallerthe optical input but the higher the probability that one or morefingers contacts are still intact after the processing. For practicalreasons, the number of fingers 146 a, 146 b typically provided duringprocessing of a pixel in a detecting device is about two to three. It ispreferred that the insulating layer 130 remains to a large extent underthe fingers. In another preferred configuration one or more fingers areconnected to an open window contact, e.g. an open circular contactrather in the centre of the pixel window. When a maximum optical inputpower is required, an anti-reflection coating 210 can be provided at thewindow opening of the pixel as illustrated in FIG. 4 c. Theanti-reflective coating 210 can be any type of anti-reflective coating,such as a stack of thin films having an alternating refractive index.Typical thin films used therefore are, e.g. coatings of ZnSe, ThF4, YF3,GaAs, Ge. In an alternative design both local electrodes 147, e.g.formed by finger contacts as described above, and a perimeter contact148, electrically separated from each other are used. The latter isshown in FIG. 4 d, showing a top view of a lay-out for a single pixelhaving a local electrode for example being a centre electrode and aperimeter electrode. The local contacts 147 thereby typically areprovided in or near the centre of the sensor. By providing both contacts147, 148 separated from each other, different types of information canbe obtained, by selecting one of these contact 147, 148 or by measuringboth an electrical signal between the local electrode 147 and areference electrode, e.g. bottom electrode, and an electrical signalbetween the perimeter electrode 148 and a reference electrode, e.g.bottom electrode 140. Both electrical signals are representative fordifferent types of information about the incident beam, such asdescribed in the first embodiment for different pulse lengths of theincident beams. For pulse lengths between the timing parameters τ_(mix)and τ_(erg), the obtained information for a local contact 147, e.g. acentral contact, of the pixels is the energy incident at the local areaof the pixel where the contact is made, e.g. in the centre of the pixelwhereas a perimeter contact 148 measures the total energy incident onthe window 110. For pulse lengths larger than τ_(pow) using a localelectrode 147, such as a centre electrode, allows to obtain informationabout the local power incident on the local area near the localelectrode 147, e.g. the centre of the pixel if a centre electrode isused, whereas for a perimeter contact, the total power of the incidentbeam on the pixel window is measured. The latter system can be usedespecially in large single pixel sensors, with a window defining anactive or sensitive region of the sensor element that is substantiallylarger than the beam size. With substantially it is meant that a typicalsize of the beam, e.g. the beam diameter, is at least 1.5 times,preferably 2 times smaller than a typical size of the window, e.g. thediameter. The perimeter electrode may furthermore also acts asprotection layer, thereby defining the active or sensitive area. In thiscase, the perimeter electrode 148 may be an electrode layer that isinsulated from the semiconductor layer 120 except for a small ringdefining the window opening. Similar to the previous configurations,insulating layers and anti-reflection coatings may be applied whereappropriate. It is to be noted that perimeter contacts yield a smallersensitivity than e.g. centre contacts. It will be obvious for the personskilled in the art that although circular and square windows are shownin the sensor elements of the present invention, the shape of the windowis not limited thereto. This explains why, for beam pulses shorter thanτ_(mix), a local contact at the centre of the pixel allows to obtaininformation about the beam intensity, while using the perimeter contact,no usable results can be obtained.

In another embodiment of the present invention, the invention relates toa detecting device comprising a plurality of pixels, i.e. e.g.comprising an array of pixels. Linear arrays are particularly suitablefor applications where there is relative motion between the detectorhead and the laser beam. Two dimensional detector arrays are used in awide range of applications. The exact arrangement of such a detectingdevice can be adjusted for specific applications, i.e. the pixels maybe, but do not have to be arranged in a matrix having n columns and mrows of pixels.

In principle, any of the pixel configurations described in the previousembodiments can be used for a detecting device having multiple pixels,especially the configurations shown in FIG. 4 a, FIG. 4 b and FIG. 4 c.Nevertheless, as soon as more than one pixel is considered, i.e. if thedetecting device exists of a one dimensional array (1D) of pixels aswill be shown in FIG. 6 a or a two dimensional array (2D) as will beshown in FIG. 11, it is important to minimize the cross-talk between thedifferent pixels for a given resolution and to keep an optimumsensitivity. In other words, pixels to be used in a multi-pixelconfiguration preferably are adapted so that a low inter pixelcross-talk is obtained.

The preferred design rules for lowering the cross-talk to an acceptablelevel are dependent on the regime of operation of the detector, thedifferent operation regimes being discussed in detail in the firstembodiment. In fast laser pulse detection applications cross-talk is ofminor importance as heat cannot be transferred fast enough bythermo-conductivity processes. Cross-talk can be neglected inapplications where pulse lengths are shorter than τ_(mix).

For laser pulses longer than τ_(mix) there is sufficient time to spreadheat, the longer the pulse the more heat spreading, hence morecross-talk. In general heat is spread through the substrate from top tobottom, i.e. creating a vertical gradient, and from the centre of thepixel to the sides, i.e. creating a horizontal gradient. To minimize thecross-talk between pixels, given a required pixel pitch, three preferredtechniques can be applied. The fill factor of pixels may be lowered, thehorizontal gradient may be kept significant smaller than the verticalgradient and/or the heat flux in the horizontal direction may beblocked.

The heat spreading in each direction is proportional to the heatspreading area perpendicular to that gradient and the value of thegradient. The heat spreading areas are described by the absorptionlength L_(abs), the perimeter P_(pix) and the surface area S_(pix) of apixel. The heat spreading area for the horizontal gradients can bedefined as L_(abs).P_(pix) whereas for the vertical gradient the heatspreading area equals S_(pix). Hence minimal cross-talk can be obtainedwith a minimum ratio P_(pix).L_(abs)/S_(pix). In the case of thecircular pixel with radius R_(pix) this ratio becomes2L_(abs)/R_(pixel). Again, in order to increase the dynamic range of thedetector, partially absorbing semiconductor substrates may be used. Thedynamic range then is exponentially dependent on the ratio of thethickness of the absorbing zone/absorption length L_(abs).

This simple expression shows that cross-talk can be lowered with smallerabsorption lengths. A smaller absorption length, however, results in amore localized energy deposition, hence in higher temperature rises. Inthe case of a very small absorption length, corresponding to extremehigh doping concentrations, the local temperature rise will beunacceptable and will lead to thermal damage. Different preferredsolutions however can be introduced to find a compromise betweenreducing cross-talk and to avoid thermal damage, which can be combinedwith all of the suggested detector embodiments of the present invention.A first preferred solution exists in the provision of a heat sink 172 atthe bottom of the substrate 120 such as illustrated in FIG. 5 a. Thisheat sink 172 has a double effect: firstly it reduces the maximumtemperatures at the top side of the substrate 120, hence larger opticalpower levels can be detected in such case and secondly it enlarges thevertical gradient with respect to the horizontal gradient, which leadsto smaller cross-talk levels. Furthermore a reflective layer 174 may beprovided adjacent the heat sink.

In FIG. 5 b, the maximum surface temperature is shown as a function ofthe position of the distance to the centre of the beam. The dotted lineillustrates the maximum surface temperature for a system without heatsink, the full line illustrates the maximum surface temperature for asystem with heat sink, while the dashed line corresponds with the beamprofile, indicated by way of illustration. It can be seen that, using aheat sink, the maximum surface temperature can be reduced with a factorof about 3.40. Additionally the cross-talk at a distance of twice theradius has been decreased from 30% down to 0.2%, corresponding to animprovement of a factor 150.

In FIG. 5 c a detector is shown wherein only part 182 of thesemiconductor substrate 120 is absorbing. In FIG. 5 d reduction of themaximum surface temperature is shown for partially absorbing substrates.FIG. 5 d shows how the maximum surface temperature is reduced bydecreasing the thickness of the absorbing layer characterized by anabsorbing length L_(a) of 294 μm. Incident power density of the 1 cmdiameter laser beam is equal to 25 kWcm⁻². The results are indicated fora detector having a heat sink placed at a distance of 350 μm from thetop of the substrate. FIG. 5 d teaches that employing only a partiallyabsorbing substrate can be very effective in reducing the maximumtemperature increase. The maximum temperature change can be easilylowered with some orders of magnitude by choosing much smallerabsorption thicknesses. This is a preferred embodiment when one has todeal with extreme high optical power levels to prevent thermally inducedsubstrate damage.

In comparison with quantum well detectors, pyroelectric and bolometermembrane detectors, moderate doping concentrations, i.e. in the range0.4.10¹⁸-4.10¹⁸ cm⁻³, in n-GaAs lead to absorption lengths which are atleast more than one order of magnitude larger. Depending of the opticalpower level and pulse width, a typical range of the absorption length inthis invention is between 20 and 1000 μm, whereas in the prior art thisis of the order of micrometer of even submicron. This explains why thedetector mechanisms of this invention are preferred over other detectorsin large optical power applications. Inter pixel cross-talk in highoptical power applications can be preferentially further reduced byusing smaller fill factors, i.e. by partially diverting e.g. reflectingthese portions of the light beam which are not close to a pixel centre.Although this does decrease the vertical heat spreading area which isnot beneficial for cross-talk reduction, the overall effect is positive.Smaller exposed pixel areas allow more space between the pixels for agiven pixel pitch, which indeed is a big advantage for the cross-talkreduction. Having smaller fill factors also leads to a smaller thermalimpact of the laser beam on the substrate. Reduction of the fillingfactor is shown for a line detector and corresponding pixelconfigurations in FIG. 6 a to FIG. 8. FIG. 6 a is an illustration of asingle array detector, i.e. a line of pixels in a multi-pixelconfiguration. The detection system shown, comprises on top of thelayers of the previously described embodiments a reflective/protectivelayer 220. The detection system furthermore shows sensor element windowopenings 110 determining the active region of the detector. Thereflective/protective layer 220 may be any type of layer having a highreflection coefficient, like e.g. a metal layer. Thereflective/protective layer 220 can be e.g. made of Al, Ni, Cr, Ag, Cuor Au. Each of the detector pixels is provided with a pad for externalconnection 222, 224. In a preferred embodiment, the external connectionscan be alternatingly positioned on the one side and on the other side ofthe line detector, thereby improving the ease of contacting.

A first pixel structure for a single pixel in a multi-pixelconfiguration, as shown in FIG. 6 a, is shown in top view andcorresponding cross-sectional view along line A-A′ in FIG. 6 b. On topof the layers of the previously described embodiments areflective/protective layer 220, separated from the rest of thestructure by an electrically and thermally insulating layer 230, isprovided.

A second configuration for a single pixel in a multiple pixel devicehaving a reduced thermal cross-talk between different pixels is shown inFIG. 7 a both in top view and in two corresponding cross-sectional viewsalong lines A-A′ and B-B′. The pixel has a similar design as the pixeldesign of the previous configuration, but in contrast to thisconfiguration shown in FIG. 6 a and FIG. 6 b, the reflective/protectivelayer 220 is not provided on top of a second insulating layer 230, butis deposited on the first insulating layer 130 such as the contactinglayer 145, but complementary thereto. This complementarity is shown inmore detail in an alternative design for a detector lay-out with areflective/protective layer complementary to the metallization paths inFIG. 7 b.

The complementarity between the reflective/protective layer 220 and thecontacting layer 145 can also be implemented by using an additionalinsulating layer 240 separating and insulating the reflective/protectivelayer 220 from the contact metal 145. The latter is shown in top viewand for two corresponding cross-sectional views along lines A-A′ andB-B′ in FIG. 8.

As a rule of thumb in the energy sensing regime, the interpixel distanceP, also referred to as interpixel pitch, should be minimal 1.5 to 2times larger than the window opening w. An illustration of a detectorwith sensor elements having a significant large interpixel pitch P isillustrated by way of example in FIG. 9.

In the power sensing regime, heat spreading between the pixels ismaximum due to the long exposure of the pixels to the laser beam.Reduction of cross-talk therefore is even more important for thisoperation regime. It is therefore preferred to obtain as much reductionas possible. Therefore, especially for detector elements in the powersensing regime, a combination of techniques is used, combining theimplementation of small fill factors with blocking of the thermal heatflux in the horizontal direction.

An illustration of a system, allowing to block the thermal heat flux inthe horizontal direction is shown in FIG. 10. If a small fill factor isused, there is sufficient space between the exposed areas to etch airchannels 275 with width e and depth d. These air channels 275 will actas good thermal insulator between the pixels. This is very effective assoon as the etch depth d is at least a substantial fraction, i.e. atleast 10% preferably 20% more preferably 50% or is at least equal to theabsorption length. Typically the width of these vias is less important.A typical width may be in the range 5 to 50 μm wide. Simulations showthat the cross-talk can be reduced with a factor 5 to 25 such that theabsolute cross-talk is below a 5% acceptance level of the laserapplication.

Persons skilled in the art will understand how to interpret detector andpixel outputs and their time dependent cross-talk influences based onthe given detector dimensions such as substrate material, windowopening, absorption length and derive a first estimate of the beamcharacteristics.

By way of illustration, exemplary results for temperature distributionand thermal cross talk are shown in FIG. 11 a to FIG. 13 b for sensorelements according to the present invention. FIG. 11 a and FIG. 11 b,illustrate exemplary graphs of the temperature distribution in the depthof the substrate for a 1 mm wide incident laser beam with a peakintensity of 25.6 kW/cm² and a pulse width of 300 μs on an n-type GaAsdetector substrate with a doping concentration N=2.5×10¹⁸ cm⁻³ and acorresponding absorption length of 46.7 μm and having a circular pixelwith a radius of 25 μm and a reflective Au layer on top of a secondaryinsulator layer. The different curves in FIG. 11 a show the temperatureevolution at the centre of the detector pixel at various depths insidethe substrate, i.e. the temporal behaviour of the temperaturedistribution. The response, i.e. temperature evolution, is shown for adepth z=0 μm by curve 302, for z=50 μm by curve 304, for z=100 μm bycurve 306, for z=200 μm by curve 308 and for z=400 μm by curve 310. Thegraph of FIG. 11 b shows the curve 312 indicating the correspondingmaximum temperatures at various depths. These exemplary graphs showwhich distance needs to be respected in the depth, to place the groundcontact such that a maximum output is detected. The ground contact thusneeds to be place outside the integrated in time absorption volume toobtain a maximum output. For the present example, a depth of at least0.2 mm needs to be respected.

FIG. 12 a, FIG. 12 b and FIG. 12 c illustrate exemplary graphs of thetemporal evolution of the temperature distribution on the top side ofthe substrate for a 1 mm wide incident laser beam with a peak intensityof 25.6 kW/cm² and various pulse widths, i.e. 300 ns, 30 Ps and 5 ms onan n-type GaAs detector substrate with an absorption length of 100 μmand having a circular pixel with a radius of 25 μm and a reflective Aulayer on top of a secondary insulator layer. The curves on the variousgraphs correspond to different distances from the pixel centre,expressed in multiples of the pixel radius, i.e. 0×, 1×, 2×, 4×, 6×, 8×,and 10× the pixel radius. In the energy sensing regime, illustrated inFIG. 12 a using a laser beam with a pulse width of 300 ns, thedetector's output is the integration of optical input. Only in thecentre of the pixel, indicated by curve 320, and at the pixel radius,indicated by curve 321, the temperature is changing. For the curve 322indicating the temperature in a point 2 times the pixel distance awayfrom the pixel centre, and the other curves (not referred to) indicatingpoints even further away from the pixel centre, the temperature isalmost constant. Once the pulse is finished, the temperatures graduallydecrease due to thermal leakage of the pixels. In the mixed energy-powersensing regime, illustrated by FIG. 12 b 12 a using a laser beam with apulse width of 30 μs, a much bigger temperature change at variousdistances from the pixels can be seen. FIG. 12 b shows the results forthe centre of the pixel, indicated by curve 330, and for points 1×, 2×,4×, 6×, 8× and 10× the pixel radius away from the centre of the pixel,indicated respectively by curves 331, 332, 333, 334, 335, 336. Thistemperature change is not any longer proportional to the incoming energydue thermo-conductive effects of the substrate lattice. The mixedenergy-power regime is characterised by the fact that at differentdistances from the pixel the temperature reaches its maximum at anothermoment in time. In this regime a numerical models needs to be applied toderive the spatio-temporal information about the laser beam. FIG. 12 cillustrates the temperature change for the power sensing regime using alaser beam with a pulse width of 5 ms. In this regime the maximumtemperature is simultaneously reached at various distances from thecentre of the pixel. The temperature information in the centre of pixelyields direct the temporal evolution of the power incident on the pixel.The temperature change is shown for the pixel centre, indicated by curve340, and for points 1×, 2×, 4×, 6×, 8× and 10× the pixel radius awayfrom the centre of the pixel, indicated respectively by curves 341, 342,343, 344, 345, 346.

Examples of cross-talk calculations where the blocking of the horizontalthermal heat flux is not implemented are shown in FIG. 13 a and FIG. 13b for the energy sensing and power sensing regime, respectively. Bothfigures give an overview of the cross-talk at the moment when thetemperature is maximum in the centre of the pixel for differentabsorption coefficients expressed in mm⁻¹. The sensor structure andlaser beam used in the example of FIG. 13 a is identical to the sensorstructure and laser beam used in the example of FIG. 12 a. FIG. 13 ashows that in the energy sensing regime the cross-talk between pixels isbetween 5% and 1% (5%-1%) when the pixel centres are 3 pixel radii awayfrom each other. This means that for an illuminated first pixel leadingto an output signal S, a neighbouring pixel, not illuminated directly,may indicate a signal of maximally 5% of the signal S measured in thefirst pixel. Obviously, if the pixel centres are separated by a largerdistance, the cross-talk is smaller. The amount of cross-talk isindicated for α=34 mm⁻¹, α=55 mm⁻¹, α=96 mm¹, α=215 mm¹, α=474 mm⁻¹ bycurves 350, 351, 352, 353, 354 respectively. The largest cross-talkvalues correspond with the smallest doping concentration, whereas thesmallest cross-talk values correspond with the largest dopingconcentration. The laser beam and sensor structures used forillustrating cross-talk in the power sensing regime as shown in FIG. 13b, is the same as that of FIG. 12 c. The curves are parametrized invalues of the absorption coefficient. The curves for the power sensingregime show that the cross talk without air gap is unacceptably higheven at large distances: more than 10% at 6 to 8 pixel radius away. Theamount of cross-talk is indicated for different absorption lengths, i.e.for α=34 mm⁻¹, α=55 mm⁻¹, α=96 mm⁻¹, α=215 mm⁻¹, α=474 mm⁻¹ by curves360, 361, 362, 363, 364 respectively. FIG. 13 c shows an example of howthis cross-talk can be substantially reduced by etching a thermallyinsulating air gap in the case of a 50 μm wide incident laser beam witha peak intensity of 25.6 kW/cm² and a pulse width of 300 μs on an n-typeGaAs detector substrate with a doping concentration N=7.7×10¹⁷ cm⁻³ anda corresponding absorption length of 294 μm and having a circular pixelwith a radius of 25 μm. Curve 370 shows the temperature change asmeasured in the centre of the light beam, while curve 371 shows thetemperature change as measured in a neighbouring pixel outside the lightbeam, separated from the pixel by an air gap of 50 μm wide. Thecross-talk is indicated by curve 372, indicating that the cross-talk canbe decreased below a 5% acceptance level when the etch depth is 50 μm,which is about 20% of the absorption length in this illustrativeexample. Table 2 shows with numerical examples how the cross-talkreduction factor depends on the ratio between the absorption length andthe depth of the thermal insulating gap. In both cases a 150 μm deep gapis compared with a flat structure. Reduction factors between 15 and 25can be reached when the gap depth—absorption length ratio is variedbetween 16% and 300%. The cross-talk reduction is most effective for thesmallest absorption lengths, but actually does not depend much on it. Ontop of that the sensitivity of the pixel has increased by about a factorof three. TABLE 2 d (μm) ΔTc (K) ΔTout (K) X-talk (%) Reduction α = 34cm⁻¹ 0 11.46 3.61 31.50 150 40.34 0.79 1.96 16.09 α = 215 cm⁻¹ 0 43.2710.26 23.71 150 123.57 1.27 1.03 23.07

It will be obvious that the results shown above by way of illustration,which are based on the above mentioned equations and the correspondingmodel, are only exemplary for the given pixel structure and that similarresults can be obtained for any incident laser beam on any suitablepixel structure on a preferred substrate of a detecting device accordingto the present invention.

In a further preferred embodiment, a detector 400 comprising a twodimensional array of pixels 402 is described, as shown in FIG. 14 a. Anelectronic scheme of a single pixel structure 402 of a 2D array isillustrated in FIG. 14 b. Although the invention is not limited thereto,the 2D-array may consist of n rows and m columns. Each pixel 402generates its own voltage V_(t,oc) according to its optical input. Eachpixel may be separately addressable so as to be able to read out thevalue on each pixel independently. Accordingly, a means for separatelyaddressing each pixel and a means to read out the value on each pixelindependently is provided. Due to the finite dimensions of the metalcontact to the pixel a local resistance 410 is present. During a giventime period all pixels of the array store their input values in astorage element 420 by means of switch 430. The storage element 420 maybe e.g. a capacitor, The read_switch 430 of each pixel is activated by acommon read_signal which is directed to every pixel during the readperiod. Before the optical pulse is incident, every switch 430 of eachpixel element is activated by a read_signal. During the read_period allread_switches remain conductive and all read_out switches remain open,i.e. in a non conductive state. After the laser pulse reading cycle isterminated, every switch 430 of each pixel is deactivated. Afterwardsthe read_out cycle is started. During the read_out cycle each rowcomprising M columns of the 2D pixel array, is step by step addressed byactivating the M column_pixel switches 440 to read out the informationfrom the storage element. When N rows of pixels are available Ndifferent read_out steps are executed in series during a total frameperiod. Other ways of read-out steps are not limiting for the scope ofthe present invention and well known by a person skilled in the art.

The read-out electronics for detecting devices comprising detectorelements as described in any of the previous embodiments are now furtherdescribed in more detail. FIG. 15 and FIG. 16 show preferredconfigurations of the block diagrams of the read-out electronics fordetecting devices. The read-out electronics can be applied for everypixel described above in accordance with the present invention. Theelectronic scheme of FIG. 15 is suitable for the digitalisation of thetemporal evolution of the beam characteristics. The clock 450 triggersin a regular way the Sample/Hold circuitry 460 where after theAnalogue/Digital 470 conversion can take place. The Sample&Holdcircuitry 460 can be any circuitry that allows obtaining a measurementresult and storing it, as described in FIG. 10 b. The Sample&Holdcircuitry 460 comprises a storage element, which can be any type ofstorage element such as e.g. a capacitor. Once the signal is digitizedit can be used for further processing for the reconstruction of thetemporal shape of the laser beam. The electronic scheme of FIG. 16 israther suited for measuring the maximum signal at the pixel output. Thepeak detector circuitry 480 records the maximum value of the temporalevolution of the laser beam incident in the particular pixel. At themoment the maximum signal is detected the Sample and Hold circuitry 460is triggered. After this process the digitalisation of the analoguesignal can be effectuated by means of the ADC circuitry 470. The abovedescribed read-out electronics can be made of standard electroniccomponents, well known by a person skilled in the art.

A further embodiment describes methods for measuring a temporal andspatial profile of a laser beam according to the present invention.Schematic illustrations are shown in FIG. 17 and FIG. 18. FIG. 17illustrates a first preferred configuration, wherein a system forgenerating an irradiation beam 510, e.g. a laser system 510, generates afirst irradiation beam 520, sampled, by a suitable beam sampler 530. Themethod and system is especially useful for high power laser systems suchas e.g. a Nd:YAG or a CO₂ laser, and splits the first light beam 520into a second light beam 540 and a third light beam 560. The opticalpower in the second light beam 540 is much larger than the optical powerin the third light beam 560. The beam sampler 530 can be e.g. a lowabsorptive beam splitter suited for high energy beams like e.g. anappropriate coated ZnSe beam splitter a leaking mirror or anacousto-optic beam splitter . . . The second light beam 540 is used forthe specific application 550 of the user. This may be one application orthe second light beam 540 may be split further for several otherapplications. The third light beam 560 is used as input for a detectoror camera 570 which is used to inform about the status of the beam 520emitted by the laser 510. The detector or camera 570 may comprise one ora plurality of any of the detector elements as described in the previousembodiments. This information about the status may comprise temporal andspatial information about the output of the laser system 510, andindirectly about the temporal behaviour of the first light beam 520profile.

In a second preferred configuration the beam sampler 530 is replaced bya mirror 580 which is attached to a rotating point 590. The mirror 580is installed such that at a given moment in time input beam 520 isreflected towards the 1D or 2D camera system 570. Also in the presentconfiguration, the detector or system 570 comprises one or a pluralityof any of the detector elements as described in the previous embodimentsThe mirror rotation is only actuated from time to time such that theinterruption of the input beam 520 for the user application 550 is onlyvery limited. This embodiment is especially useful for continuousworking lasers. For pulsed laser systems, the timing of rotation of themirror 580 can be adapted to the timing of the laser pulses so as tosynchronise with the pulse frequency.

For high power laser systems, the fraction of the laser beam that isguided towards the camera is limited preferably to some percentage. Inthe case of pixels of 25 μm radius pixels, a GaAs substrate detector,the substrate temperature should be limited under the 800 K.

It is to be understood that although preferred embodiments, specificconstructions and configurations, as well as materials, have beendiscussed herein for devices according to the present invention, variouschanges or modifications in form and detail may be made withoutdeparting from the scope and spirit of this invention.

1-24. (canceled)
 25. A detector for characterising a high power lightbeam, said detector comprising at least one pixel, wherein each of saidat least one pixel comprises a doped semiconductor substrate adapted tosubstantially absorb an incident light beam, being at least a fractionof said high power light beam, over an absorption distance L, saidsemiconductor substrate having thermoelectric properties, said dopedsemiconductor substrate being doped over at least part of the thicknessof the semiconductor substrate; means for partly covering the dopedsemiconductor substrate such that a window opening is provided at afirst side of the doped semiconductor substrate for receipt of saidincident light beam in said doped semiconductor substrate; and means formeasuring an electrical signal induced by a thermoelectric effect insaid doped semiconductor substrate comprising a first electrode at saidfirst side of said doped semiconductor substrate and a referenceelectrode, said reference electrode positioned outside an absorptionvolume determined by said window opening and said absorption distance L.26. A detector according to claim 25, the detector operating accordingto an absorption principle, wherein the absorption principle is a freecarrier absorption process.
 27. A detector according to claim 25, thedetector comprising a means for reducing cross-talk.
 28. A detectoraccording to claim 27, wherein said means for reducing the cross-talk isa means for reducing the cross-talk between neighboring pixels of saidplurality of pixels adapted for reducing the cross-talk at least below a5% acceptance level.
 29. A detector according to claim 27, wherein saidmeans for reducing the cross-talk is a means for reducing the cross-talkbetween any pixel electrode of said plurality of pixels and thereference electrode, adapted for reducing the cross-talk at least belowa 5% acceptance level.
 30. A detector according to claim 27, whereinsaid means for reducing the cross-talk comprises a cooling channelbetween at least two pixels.
 31. A detector according to claim 27,wherein the means for reducing cross-talk are positioned outside a planedetermined by said semiconductor substrate.
 32. A detector according toclaim 27, wherein the means for reducing cross-talk comprises a heatsink provided in thermal contact with said semiconductor substrate. 33.A detector according to claim 27, the means for reducing cross-talkcomprising a reflector material positioned around the window opening.34. The detector according to claim 25, wherein the means for partlycovering the doped semiconductor substrate and said absorption distanceL are adjusted so that $\frac{P_{pixel} \cdot L}{S_{pixel}\quad}$ is inthe range 0.1 to 100 wherein P_(pixel) is the perimeter of the pixelwindow and S_(pixel) is the surface area of the pixel window.
 35. Adetector according to claim 25, said characterising being detecting aprofile selected from the group consisting of at least one of a spatialand temporal intensity profile, a spatial energy profile, a spatialenergy density profile and at least one of a spatial and temporal powerprofile of the beam.
 36. A detector according to claim 25 wherein saidfirst electrode defines at least the perimeter of the window opening.37. A detector according to claim 25, wherein said first electrodefurthermore comprises at least one elongate electrode extending over thewindow opening.
 38. A detector according to claim 25 wherein said firstelectrode is separated at least partly from said doped semiconductorsubstrate by means of an insulating layer.
 39. A detector according toclaim 34, wherein the adjustment of the absorption distance L isperformed by adjusting the doping level of said doped semiconductorsubstrate.
 40. A detector according to claim 25, comprising a pluralityof pixels, each pixel having a pixel window with an average pixel windowwidth w, the pixels being separated by at least an interpixel pitch P,wherein said interpixel pitch P is between 1 and 10 times the averagepixel window width w.
 41. A detector according to claim 25, wherein ateach first electrode a switch and a storage means is provided, fortemporary storing the pixel information.
 42. A detector according toclaim 25, wherein said detector system furthermore comprises a read-outelectronic circuitry adjusted to sample at regular moments the timeevolution of the electrical detector outputs and convert the sampledanalog voltages into digital signals.
 43. A detector according to claim25, wherein said second electrode is positioned at a second side of thecompletely doped semiconductor substrate, the first and second sidebeing opposite with respect to each other.
 44. A system for monitoringthe output of a light beam producing means comprising a light beamsampling means and a detector, wherein said light beam sampling means isadjusted to split the light beam in a first small fraction and a secondlarge fraction, and wherein said first small fraction of said light beamis directed towards a detector according to claim
 25. 45. A system formonitoring according to claim 44, wherein said beam sampling means is amirror, said mirror being rotatably mounted as to split the light beamat regular periods.
 46. A method for measuring the optical power of ahigh power incident radiation beam, the method comprising: receiving anincident light beam, being at least a fraction of the high powerincident radiation beam, in a doped semiconductor substrate through awindow opening provided at a first side of the doped semiconductorsubstrate; absorbing the incident light beam in the doped semiconductorsubstrate being doped over at least part of the thickness of thesemiconductor substrate; and measuring at least one electrical signalinduced by a thermoelectric effect in the doped semiconductor substrateusing a first electrode at a first side of the doped semiconductorsubstrate and a reference electrode outside an absorption volumedetermined by the window opening and an absorption distance of theincident light beam in the doped semiconductor substrate, wherein theabsorption length L_(abs) for the light beam in said doped semiconductorsubstrate may furthermore be adjusted to keep the temperature of thesurface of said doped semiconductor substrate in said window openingbelow a maximum temperature.
 47. A method according to claim 46, whereinsaid measuring comprises measuring at several points in the crosssection of the fraction of the incident light beam by subsequentlyshifting a single or multiple pixel detector, as described in claim 25,to another point in the cross-section of the fraction of the light beamand recording a measurement.
 48. A method according to claim 46, saidincident light beam having a pulse width and said detector being capableof measuring in intensity measuring mode, in energy measuring mode, inenergy density mode or in power measuring mode, the method comprisingsetting a pulse width of said light beam for measuring in a modeselected from the group consisting of intensity measuring mode, energymeasuring mode, energy density mode and power measuring mode.
 49. Adetector according to claim 25, comprising a plurality of pixels, eachpixel having a pixel window with an average pixel window width w, thepixels being separated by at least an interpixel pitch P, wherein saidinterpixel pitch P is between 1.5 and 4 times the average pixel windowwidth w.